Fault finding issue help please

[saint]

I have installed two strings of eight panels on a single story building. The top row of eight seem to be performing well. But the bottom string performs only about half to two thirds of the top.

Midday results yesterday which was sunny .......
String 1 240v and 1113w
String 2 261v and 493w

I'm not sure why the v are higher on string 2? Maybe a current issue?

There is some minor shade from a hedge early and late but not at midday. I have tested each of the eight panels this morning and each Voc and isc are much the same.

Advise would be very welcome.

Good job were not busy!

 

[saint]

11am today
String 1. 268v 1.4a 250w.
String 1. 268v 0.7a 220w.
115 w/m2

 

[sharpener]

Weird. Yesterday's results look consistent with String 2 being run at current lower than the correct MPP and hence the voltage is higher. Today's results are even odder, why would half the current produce nearly as much power?

As you have separate measurements I assume you have got a dual input inverter, try swapping the strings at the inverter to rule out an inverter or MPPT fault.

Some more details of the system (panels and inverter type) might help others with more specific experience of the products to chip in.

 

[saint]

have ruled out mppt fault as you suggest.
16x schott panels with power one pvi 3.6

 

[Graeme Harrold]

Check your connections, pain in the rear I know. Was there any reason you wired the panels on 2 rows rather than 2 blocks of 8 (4x2), this keeps the cabling slightly shorter....

 

[saint]

Today we changed the array from 2 rows of 8 to 2 blocks of 4. Client was certain there wasn't any shading issue from a hedge on the bottom row of 8 panels but we couldn't find any other reason for the lower performance. left the job with 100w and 95w on the strings this afternoon which is closer than this morning (76w and 58w) when 2 rows of 8. We will see what is generated over the next few days.

 

[sharpener]

Ironically if it turns out there is shading on the bottom you may want to keep it as two rows of 8 so at least the unshaded part can operate properly.

 

[saint]

yes i realise that scaffolding doesn't help either! client is cutting more off the hedge this week hopefully. Fault finding pv in winter light is not ideal!

 

[Earthstore]

Had a similar issue on a 30KW system, split on two 15000tl inverters, lower half of roof was on one and upper half on the other.
The top half was producing 6500kw, and the bottom half was producing 4200kw,
ran up on the roof and as the sun was low in the sky the scaffold was was casting a shadow on the lower strings.
When the scaffold was removed, both were producing 7500kw when I checked, not to bad for winter..
It is amazing how a little shading can drag a system down, something that some people ignore.

You may well find it's OK when the scaffold is removed.

Hope this helps

 

[moggy1968]

If a connection was at fault you would have no output, it's all or nothing with connections.